1. Field of the Invention
The present invention relates to a plasma processing apparatus which turns a source gas into a plasma and processes the surface of a solid material such as a semiconductor by physical or chemical interaction of activated particles.
2. Description of the Related Art
It is known that there are plasma processing apparatuses as given below which are designed to generate plasmas with improved performance (uniformity, stability, etc.) for processing semiconductor material surfaces.
JP-A No.111996/1994 discloses a method in which N electrodes are rotationally symmetrical and RF (13.56 MHz) power is impressed on each of the electrodes, with the phase varying in steps of 360 degrees/N.
JP-A NO. 135438/1999 describes a method in which linear conductors are positioned radially from a center and grounded at their outer ends and RF power is impressed on them.
JP-A No.125663/1998 reveals a method in which RF power is impressed on conductive windings extending spirally from a center.
JP-A No.111996/2000 also discloses a method in which UHF high frequency power is impressed on each of rotationally symmetrical N antenna elements whose length is a quarter of the wavelength of the high frequency power to be impressed, with the phase varying in steps of 360 degrees/N.
JP-A No.70108/1998 suggests a method in which UHF high frequency power is respectively impressed on radially extending bar conductors with a length equivalent to a quarter of the wavelength of the high frequency power to be impressed, and on bar conductive antennas extending from a peripheral ring-like conductor toward the center, with a phase difference of 180 degrees.
In connection with processing with a plasma, conventional methods for discharge in a frequency band of several megahertz to 100 MHz include parallel plate capacitive coupling and inductive coupling such as ICP which uses loop coils.
According to Paschen's law which indicates the relation between the product of pressure and inter-electrode distance and dielectric breakdown voltage, in a pressure range required for an etching process, the lower the pressure is, the higher discharge maintenance voltage is required (S. Kakuta, et.al., Jpn. J. Appl. Phys. 33 (1994) pp.4335-4339).
Therefore, in the above frequency band (several megahertz to 100 MHz), it is difficult for the parallel plate method to maintain a homogeneous, high density plasma in the low pressure range and the process window in the low pressure range is restricted.
In the inductive coupling method, an induction field is generated in a plasma by applying an electric current to a loop antenna to induce a magnetic field. Contrary to the parallel plate method in which the locus of electrons is determined by the inter-electrode distance, electrons move circularly in the inductive coupling method; therefore, in principle, electrons' moving distance is longer and a higher plasma density can be obtained at low pressures in the inductive coupling method than in the parallel plate method.
However, when a low frequency band of several megahertz to 100 MHz is used for a high frequency power supply, a strong sheath electric field is generated inside the plasma beneath the loop antenna and high-energy ions go into a dielectric window around the area beneath the loop antenna, causing wear of the dielectric window.
On the other hand, when a frequency band of over 100 MHz is used, electrons are trapped in an electric field which varies with time, and thus loss due to diffusion of electrons is reduced; as a consequence, it is possible to maintain the plasma more stable even in a low pressure range and provide a wider process window to deal with a lower pressure range than when a frequency band of 100 MHz or less is used (S. Kakuta, et.al., Jpn. J. Appl. Phys. 33 (1994) pp.4335-4339).
In addition, the sheath electric field is weaker at 100 MHz or more and the energy of ions implanted into the dielectric window decreases, which reduces wear of the dielectric window (T. Kitajima, et al., Appl. Phys. Lett. 77 (2000) pp.489-491).
If we also count discharge methods which use a frequency band of over 100 MHz, in addition to the conventional parallel plate method and inductive coupling method, we should include the following: a method (phase control type) in which electric power with different phases is impressed on a plurality of antenna elements; and a method (antenna length control type) in which the antenna length is an integral multiple of a quarter of the wavelength of the high frequency power to be impressed.
Even when a frequency band of over 100 MHz is used, if the electric field in the inter-electrode direction is the main component of the plasma generating electric field as in the parallel plate method, the component perpendicular to the plasma interface is shielded as the plasma density increases, and the process window in the high density range is restricted. On the other hand, for expansion of the process window to the high density range, it is desirable to use the inductive coupling method which generates a loop induction field from a magnetic field induced by a loop current. However, if a frequency band of over 100 MHz is applied to a conventional loop antenna, a standing wave is excited inside the antenna and a loop induction field cannot be generated. It is also impossible to generate a loop induction field by a method which impresses high frequency power with different phases on a plurality of antenna elements.
Regarding a plasma processing apparatus for use in the manufacture of semiconductors, there is demand not only for an apparatus which can process large-diameter wafers with a high processing uniformity but also for an apparatus which is suitable for the following various processes: a fine etching process with a high anisotropy or selectivity ratio for gate electrodes, metal films and insulating films; an anti-reflective coating process before etching such as BARC (Bottom Anti-reflective Coating) or BARL (Bottom Anti-reflective Layer); a process for making a hard mask as an oxide film or nitride film; a thinning process for controlling the mask size; a trenching process in which the angle, radius or the like is controlled; a post-treatment process which removes etch residue and damaged layers; a sputtering process and so on.
Just taking an etching process concerning formation of transistor gates as example, the process consists of many steps including trenching, anti-reflective coating, mask making and mask thinning, gate formation and subsequent spacer formation. For throughput improvement and prevention of deterioration due to the atmospheric air, the apparatus is expected to be able to carry out all these steps.
In the etching process for formation of wiring layers, a thick upper metal film should be made at high speed for multi-layered wiring. The metal film making step also includes anti-reflective coating, anti-diffusion coating and masking. As in the gate electrode formation process, an apparatus which is capable of carrying out all these tasks is demanded here.
In addition, recently, demand for small-lot diversified production has been growing and the use of wafers which carry various device structures such as system LSIs has been spreading. In order to meet such demand, it is necessary to generate a highly uniform plasma which can be used to treat large-diameter wafers in the following wide processing ranges: a processing pressure range from 0.1 Pa-10 Pa for various seed gases and an ion implantation current range of 0.3 mA/cm-3 mA/cm2 for wafers.
In order to ensure stable, continuous plasma generation to process large-diameter wafers uniformly using a wide range of seed gases under wide-ranging density and pressure conditions, preferably a UHF (100 MHz-3 GHz) high frequency power supply should be used to generate a low-dissociation plasma at low electron temperatures and a wider process window is needed. However, in the conventional inductive coupling method which uses an RF power supply (13.56 MHz), when UHF power is supplied to a loop antenna, a standing wave is excited in the loop antenna and a loop current can not be generated, so an induction electric field cannot be generated, resulting in a failure in generation of a high density plasma.